1. Field of the Invention
This invention relates to load failure detection in the drive unit of an electric load and, particularly, to a load short-circuit failure detection method and apparatus for an electric power steering device and the like.
2. Prior Art
Heretofore, as a load short-circuit failure detection method for an electric power steering device, as disclosed in Laid-open Japanese Patent Application No. Hei 2-162159, for example, there has been used a method in which current detection means is provided in a motor drive circuit to detect a drive current (load current) for the motor and it is judged whether a steering motor as a load is short-circuited by judging whether the detected current value is within a predetermined range. Laid-open Japanese Patent Application No. Hei 3-256514 discloses a method for detecting a short-circuit state of an electric drive actuator which is an inductive load like the above motor, in which load current detection means is provided in an actuator drive circuit to detect a current of the above load and the short-circuit state of the actuator is detected by comparing the detected current value with the predetermined smallest value and largest value of drive current.
However, in the above conventional method for detecting a load short-circuit failure, for instance, even when an excessive load current runs temporarily by quick steering operation or the like, it is erroneously detected that the motor (or the actuator) is short-circuited and the apparatus is stopped erroneously in spite of the fact that there is no abnormality in the apparatus.
Laid-open Japanese Patent Application No. Sho 61-169366 discloses a method for preventing the above erroneous detection by setting a short-circuit decision time when a short-circuit failure of the load (motor) of an electric power steering device is to be detected. FIG. 13 is a diagram showing the configuration of the above electric power steering device. In the figure, reference numeral 1 denotes a steering wheel, 2 a torque sensor for detecting a steering torque, 3 tires, 4 a motor as an electric load to be detected for its short-circuit failure, 5 a power source for supply a current to the motor 4, 6 load current detection means for detecting a current running through the motor 4, 7 load current control means for controlling a motor drive current (load current) based on the steering torque detection value of the torque sensor 2 and the load current detection value of the load current detection means 6, and 8 load drive means for driving the motor 4 as a load in accordance with the output of the load current control means 7. In the figure, reference numeral 9 represents load short-circuit failure judging means for judging a short-circuit failure of the above motor 4 and taking a fail-safe action for cutting off power supply to the motor 4 when it judges that a short-circuit failure occurs in the motor 4, short-circuit current judging means 10 for judging whether the load current detection value of the load current detection means 6 is larger than a predetermined current value which is considered as a short circuit of the load, 11 a microcomputer which stores a program for judging a short-circuit failure, and 12 power switch means for cutting off power supply from the power source 5 to the motor 4 when the load short-circuit failure judging means 9 judges that a short-circuit failure occurs in the motor 4.
A description is subsequently given of a load short-circuit failure detection method in the above configured electric power steering device. A load current running through the motor 4 driven by the load drive means 8 is detected by the load current detection means 6 and applied to the short-circuit current judging means 10. The short-circuit current judging means 10 judges whether the applied detection value of the load current is larger than a predetermined value (short-circuit current decision value) and applies the result of this judgment to the microcomputer 11. Since an excessive load current (short-circuit current) runs through the motor 4 when the motor 4 is driven in the state of a short-circuit failure, the above short-circuit current decision value is set to a value equivalent to a short-circuit current, whereby the microcomputer 11 can detect a short-circuit failure. At this point, the microcomputer 11 measures a time during which the above short-circuit current continues and judges that a short-circuit failure occurs in the motor 4 when this continuation time exceeds a predetermined time (short-circuit decision time). The microcomputer 11 opens the power switch means 12 when it judges that a short-circuit failure occurs in the motor 4 and takes a fail-safe action for cutting off the connection between the power source 5 and the motor 4. On the other hand, when the detection value of the load-current falls below the short-circuit decision value before the passage of the short-circuit decision time, the microcomputer 11 judges that the above excessive load current is a transient large current caused by a sudden change in the target current and not by a short-circuit failure and keeps controlling electric power steering without taking a fail-safe action.
However, since the conventional load short-circuit controller is, as described above, configured such that it feeds back a load current detected by the load current detection means 6 to the load current control means 7 to control the drive current of the motor 4 in order to protect the load drive means 8, a load current (short-circuit current) which runs into the motor 4 at the time of a failure such as a short circuit of a load or the grounding of a wiring for a load oscillates. Therefore, as the amplitude of the above short-circuit current does not simply increase but changes around the above short-circuit current decision value, the measured short-circuit current continuation time does not reach the above short-circuit decision time. Therefore, the above conventional load short-circuit controller cannot detect a load short-circuit failure and hence, cannot take a fail-safe action such as alarming or the like.